Shearwater Perdix Tissues Graph

[Doctor Rig]

Is there any practical use for the “Tissues Graph“ on the Perdix. I’ve casually looked at it during surface intervals, but wonder if anyone has any experience or recommendations for its use. (There is a 2012 YouTube video that has some info, but it‘s dated.)

 

[inquis]

The skew toward either fast or slow tissues is interesting. Skewed to fast and a shorter SI is possible. Strictly qualitative. Using the planner will give more specific information.

There's also some utility when diving multiple days showing you the "head start" you have on loading. At some point, a day off is useful. (GF99 also gives similar info.)

 

[Lorenzoid]

You can look at the tissue graph not only during your surface interval but during a dive. I'm far less educated about decompression than many others on SB, but here's my take on it. For the average recreational diver, the tissue graph may not have much use, because how far you are from your NDL is apparently a good proxy for the likelihood of not getting DCS. Now, if I understand correctly, the NDL is based on the tissue loading in the leading compartment only, whereas that graph shows all 16 compartments in the Buhlmann model. As I understand it, there is growing support for the theory that when doing more aggressive diving DCS may not always track what's happening in the leading compartment alone. Rather, loading in multiple compartments, or the total gas load if you will, may also be an indicator of the likelihood of DCS, perhaps particularly the sub-clinical sort where the diver may just not feel quite right but doesn't exhibit any of the established DCS symptoms. If a diver can adjust their dives to reduce that total load, they may feel better or otherwise improve their safety margin. I likely mis-stated something, but that is how I understand one interesting (to me) aspect of the graph's potential utility.

 

[clownfishsydney]

No, no idea why they bother (as do some other computers) as it is too small to really make out and you do not really need to know this as the computer works it all out anyway.

 

[Wibble]

Its interesting to watch during ascent and decompression. As ever it shows the mid compartments controlling the decompression ceiling.

Pretty meaningless to NDL dives. Maybe interesting on the surface showing your intra-dive off-gassing.

 

[rongoodman]

When it becomes interesting to watch is on a liveaboard, with four or five dives daily for multiple days. You quickly gain a picture of which compartments are relevant, with the fast compartments clearing during the surface interval, the medium ones clearing over night, and the slow ones gradually accmulating some gas load over the course of the trip. The picture below is of my Perdix following nine days of diving(39 dives as I recall) on the Palau Agressor a couple of years back.

 

[Belzelbub]

Is there any practical use for the “Tissues Graph“ on the Perdix. I’ve casually looked at it during surface intervals, but wonder if anyone has any experience or recommendations for its use. (There is a 2012 YouTube video that has some info, but it‘s dated.)

Practical? Not sure. I have only once used it for more than just a curiosity.

A few of us were diving in the Keys. This was on the second dive of the day. Both dives were to a max depth of around 100 fsw, with average depth a bit over 60 fsw. On the second dive, my buddy ran low on air, so he had to omit his safety stop. My other buddy and I completed our stops, then headed up. LOA diver then swapped to a tank and headed down to do a safety stop. His wife was on the boat. She is not a diver, but familiar with diving. She was worried, so I pulled up the graph on mine. She relaxed a bit when she saw that the graph was well clear of the red.

 

[admikar]

I find it usefull.
When diving, I always run scenarios in my head of what would happen if need to get out NOW.
Your computer will only show your deco obligation, not what compartment is controling it.
I can use that information to calculate risk if I have to get out of the water right now. Even though DCS hit is possible, if it's one of the fast compartments, it should offgas relatively quickly. If it's one of middle compartments, that is trickier to clear without more medical care.
So far, I never had my slow compartments control my deco.

 

[beldridg]

I find it usefull.
When diving, I always run scenarios in my head of what would happen if need to get out NOW.
Your computer will only show your deco obligation, not what compartment is controling it.
I can use that information to calculate risk if I have to get out of the water right now. Even though DCS hit is possible, if it's one of the fast compartments, it should offgas relatively quickly. If it's one of middle compartments, that is trickier to clear without more medical care.
So far, I never had my slow compartments control my deco.

Note that you can also use the Shearwater SurfGF to get a good idea of what the risk is of "getting out the water now." E.g., if your SurfGF is 110%, you might be okay. If your SurfGF is 300%, you are screwed.

- brett

 

[VikingDives]

I find it usefull.
When diving, I always run scenarios in my head of what would happen if need to get out NOW.
Your computer will only show your deco obligation, not what compartment is controling it.
I can use that information to calculate risk if I have to get out of the water right now. Even though DCS hit is possible, if it's one of the fast compartments, it should offgas relatively quickly. If it's one of middle compartments, that is trickier to clear without more medical care.
So far, I never had my slow compartments control my deco.

That's an interesting way to think about it.

Personally, I find the SurfGF more useful. That number makes more sense to me than the picture, but I've never thought about it the way you are...